It is known from Canadian Patent No. 913,328 to produce chlorine dioxide and chlorine by reduction of sodium chlorate with chloride ions in an acid aqueous medium in which the chloride ions and the acidity are introduced by hydrochloric acid.
The reactions which are involved in the reduction of chlorate with hydrochloric acid are represented by the equations: EQU NaClO.sub.3 + 2HCl .fwdarw. ClO.sub.2 + 1/2Cl.sub.2 + H.sub.2 O + NaCl (1) EQU 3NaClO.sub.3 + 6HCl .fwdarw. 3Cl.sub.2 + 3H.sub.2 O + 3NaCl (2)
The process of the prior patent is carried out in a single chambered generator-evaporator-crystallizer vessel to form chlorine dioxide in continuous manner from a reaction medium held in the reaction vessel. The reaction medium is maintained at its boiling point at the prevailing absolute pressure in the reaction vessel and at a temperature below which substantial decomposition of chlorine dioxide occurs. The reaction vessel is maintained under a subatmospheric pressure to maintain the reaction medium at the boiling point and the water evaporated from the reaction medium is used as a diluent gas for the chlorine dioxide and chlorine. After start-up, by-product sodium chloride is formed and its concentration builds up until the reaction medium is saturated thereby, and the sodium chloride deposits from the reaction medium in the reaction vessel for removal therefrom after reaching saturation.
The process is generally conducted under steady state conditions in which the quantity of water introduced to the reaction vessel with the reactants is balanced by the quantity of water removed from the reaction vessel, mainly as diluent gas for the chlorine dioxide and chlorine, so that the liquid level in the reaction vessel is maintained substantially constant.
Any chlorate which is reacting by above equation (2) produces only chlorine and hence represents an inefficiency in the process. The "efficiency" of the chlorine-dioxide producing process is considered to be the extent to which sodium chlorate is converted to chlorine dioxide in accordance with the reaction of equation (1).
The efficiency may be expressed as a percentage or as the "Gram Atom Percent Chlorine Dioxide" (or G.A.%ClO.sub.2), which is a quantitative expression relating to the efficiency of conversion of sodium chlorate to chlorine dioxide by the reaction of equation (1) and refers to the quantity of chlorine atoms formed as chlorine dioxide as a percentage of the total quantity of chlorine atoms formed in a particular gas mixture. Thus: ##EQU1## It follows from equation (3) that the maximum gram atom percent chlorine dioxide attainable is 50%, which is equivalent to an efficiency of 100%.
The process of Canadian Patent No. 913,328 is less than 100% efficient, even under the preferred conditions outlined in copending U.S. application Ser. No. 770,359 filed Feb. 22, 1977 (now U.S. Pat. No. 4,075,308) in the names of Rapson, Fredette and Meyers and assigned to the assignee of the present application usually having an efficiency equivalent to GA% ClO.sub.2 values of about 30 to about 44%.